1. Field of the Invention
The present invention generally relates to the high-fidelity transmission of analog signals over electrical transmission cables and, more particularly, to a split cascode line amplifier for current-mode signal transmission.
2. Description of the Related Art
A considerable disadvantage and limitation of electrical transmission cables used to carry analog signals is the self capacitance of such cables. Here, signals carried by these cables tend to be attenuated and distorted to a degree that is determined by the impedances of the source and sink, as well as the length of the cable. In this case, signals at higher frequencies and signals sent over longer cables suffer a greater level of degradation.
Another disadvantage and limitation associated with transmission cables is that some types of insulation used in the manufacture of such transmission cables have dielectric and absorption constants that vary significantly with frequency, especially at low frequencies. This causes disturbing distortions that are difficult to eliminate by equalization or the adjustment of tone controls. Such distortions of signals at low frequencies are particularly caused by dielectric absorption, i.e., soakage. This is in addition to the previously mentioned signal attenuation and distortion of the signal at high frequencies.
Yet another disadvantage associated with transmission cables is capacitive loading of the signal source by the cable. In the case of electromagnetic pickups, such as those used on steel-stringed musical instruments, this capacitive loading reduces the resonant frequency of the electromagnetic pickups.
With sufficient cable length and thus capacitance, the resonant frequency shifts into the mid-band (e.g., 3 to 5 KHz). This shift causes an overemphasis of the higher harmonics which leads to a harsh sounding instrument. In the case of piezoelectric transducers, cable capacitance reduces both the signal amplitude and bandwidth, but does not cause resonance to occur.
It is known in the prior art to diminish the above disadvantages by choosing cables which have a relatively low capacitance per meter and are made of insulating material having relatively low dielectric absorption. However, the improvement, although significant, is not sufficient to eliminate the above mentioned problems. As a result, the length of such cables is greatly limited.
Another prior art approach for minimizing the foregoing disadvantages is to provide voltage amplification and/or ensure that the signal source has a low output impedance. This permits the effect of cable capacitance to be greatly reduced. This is achieved at the expense of increased complexity and manufacturing costs. Also, voltage amplification does not solve the effects of distortion caused by dielectric absorption.
Another disadvantage associated with using electrical transmission cables to carry analog signals is that some transmission cables are themselves microphonic; that is, they respond like microphones and pick up extraneous sounds. This results in annoying noises that occur when the cables are handled or disturbed. This is a particular problem during public performances, where it is difficult to avoid disturbing the cables. Microphonics are especially significant in coaxial cables that are used in conjunction with high-impedance sources.
In coaxial cables, microphonics are primarily due to the triboelectric effect. This is caused by the cable shield rubbing against the outer surface of the cable insulation that is located between the center conductor and the shield of the cable. In conventional systems, triboelectric effects are reduced by the use of a special cable design. In this case, a resistive layer is interposed between the outer surface of the cable insulation and the inner surface of the shield. As a result, any triboelectric charge which is generated by handling or disturbing the cable is harmlessly drained off.
Cascode amplifiers are well known and are widely used as wideband or high-frequency amplifiers, having originally been developed in the late 1930s for amplification of radio frequency signals using vacuum tubes. This traditional circuit has subsequently been adapted to use field effect and bipolar transistors.
The use of one set of conductors to carry both electrical power and signals, i.e., phantom power, is also well known and widely practiced. This dates back to the telegraph and the early days of the telephone, circa 1900.
One characteristic of cascode circuits is their use of two three-terminal amplifying devices in cascade, where a high-impedance output of a first amplifying device feeds a low-impedance input of a second amplifying device. Here, the intent is to maintain wideband performance by eliminating the Miller Effect in both amplifying devices.
All practical amplifying devices have undesired parasitic capacitances between their terminals. In an inverting voltage amplifier, such as a junction field effect transistor (JFET) used in a grounded-source configuration, a small voltage change at the gate causes a much larger and opposite voltage change at the drain. Here, the ratio of the two changes is the voltage gain of the amplifier utilizing the JFET. A reasonable magnitude of voltage gain would be at least 10. As a result, if the input voltage to the JFET changes by +0.1 volts, the resultant output change would be −1.0 volts. Here, an inverting gain is signified by the minus sign. This implies that a current flowing from the gate to the drain via the parasitic capacitance is increased by the voltage gain. As a result, if the parasitic capacitance is typically 5 picofarads, the effective capacitance is in the order of ten times larger, or 50 picofarads. This increase in effective capacitance is called the Miller Effect, and it causes the bandwidth of the amplifier to be reduced sharply. In this case, the reduction of the bandwidth is by a factor of ten.
If the above inverting amplifier is the first (i.e., common-source) amplifying device of a cascode, the very low input impedance of the second (i.e., common-base) amplifying device renders the voltage gain of the first device negligible. As a result, the Miller Effect is prevented from occurring, and the bandwidth of the first stage of the overall cascode amplifier is preserved.
In the second device, the transistor base is grounded. As a result, the voltage variation that is observed on the collector of the transistor has no effect on the current that is supplied by the first device to the second device. Hence, the Miller Effect is also prevented from occurring in the second device. This double elimination of the Miller Effect permits a cascode amplifier to have a wide bandwidth.
While current-mode transmission is not currently used in the vast majority of audio or digital data transmission systems, current-mode transmission is well known and widely used in the field of industrial instrumentation in the form of sensors having “4-20 milliamp” interfaces. Here, the sensors are powered by a constant voltage source, and can be used to indicate temperature by varying the current drawn from 4 milliamps (i.e., a minimum temperature) to 20 milliamps (i.e., a maximum temperature). The large physical size, high power, and low bandwidth of sensors with 4-20 milliamp interfaces precludes their use in carrying high fidelity, low-level audio, or data transmission signals through electrical cables. Accordingly, there is a need for a way to implement current-mode transmission that is suitable for use in audio and data transmission systems.